The design of tunnels for roads and railways often utilise separate tunnels to carry traffic in opposite directions. In this paper numerical limit analysis and semi-analytical rigid block techniques are used to investigate the effect of inter-shaft distance on the stability of two square tunnels constructed side by side. The tunnels are modelled under conditions of plain strain assuming that the tunnels are infinitely long. Bounds on the stability of the tunnels are obtained using finite element limit analysis, the numerical formulation of which is based upon the upper and lower bounds theorems of classical plasticity. Solutions are obtained using advanced conic programming schemes to solve the resulting optimisation problems. Upper and lower bound estimates on the stability of the tunnels are obtained for a range of geometries. These bounds, which bracket the true collapse load from above and below, are found to differ by at most 5%. Results from this study are summarised in stability charts for use by practising engineers.
12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG). Proceedings of the 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) (Goa, India 1-6 October, 2008) p. 4284-4291